Air conditioning system

ABSTRACT

Air conditioning system ( 10 ), in particular an air conditioning system having carbon dioxide as refrigerant, particularly for use in a motor vehicle. The air conditioning system ( 10 ) comprises a compressor ( 12 ), a gas cooler ( 14 ), an expansion valve ( 16 ) and an evaporator ( 18 ) arranged in series and forming a closed circuit for the refrigerant. The compressor ( 12 ) has a compressor capacity control element and the air conditioning system ( 10 ) further comprises a controller ( 42 ) for controlling the compressor capacity control element and the expansion valve ( 16 ) so as to regulate an expansion valve inlet pressure. The controller ( 42 ) regulates the expansion valve inlet pressure by controlling the compressor capacity control element so as to align evaporator air off temperature with a set point; by monitoring expansion valve inlet temperature; by determining a required expansion valve inlet pressure corresponding to the monitored expansion valve inlet temperature by means of a control algorithm; and by adjusting the expansion valve ( 16 ) and the compressor capacity control element together along an iso-capacity curve to the required expansion valve inlet pressure.

The present invention relates to an air conditioning system, inparticular to an air conditioning system having carbon dioxide asrefrigerant, particularly for use in a motor vehicle.

STATE OF THE ART

As a result of the need to reduce the energy consumption arising fromthe use of automotive air conditioning systems, electronicallycontrolled compressors are being increasingly applied. This permitsexternal control, which can be used to advantage in a number ofdifferent ways. The most valuable in energy efficiency terms is themanagement of the evaporation temperature to reduce the necessary amountof reheat to a minimum.

Automotive air conditioning systems with carbon dioxide as refrigerantusually have an electronically controlled compressor, but also requirean extra degree of flexibility in the form of an electronicallycontrolled expansion valve. The result of having two control elementsmeans that different combinations of settings of the two devices canyield the same cooling performance. However these different combinationswill have different energy efficiencies. A control system can be used tocontrol the system to the combination, which yields the highest energyefficiency.

The standard solution to this problem is to look for optimum COP(coefficient of performance) for fixed compressor displacement and isbased on the recognition that for any given operating point the requiredhead pressure for optimum efficiency is a simple function of therefrigerant temperature at the gas cooler outlet. The control problemreduces to one of establishing the relationship so that for any measuredgas cooler outlet temperature the system knows what head pressure tocontrol to.

An other proposed solution to the problem is given in WO 00/06821. Inthis patent application, it is disclosed that one should identify theoptimum COP for fixed system cooling capacity and not for fixeddisplacement. WO 00/06821 defines an operating band for all the COPoptima and claims that one only needs to control within this band toachieve optimum efficiency. The operating band is determined by takinginto account the refrigerant temperature and pressure on the expansionvalve inlet side. Experimental data however indicates that to it isinsufficient to lie within the operating band defined in WO 00/06821 butthat each operating point has its own tolerance band to ensure controlis near enough to the optimum. Thus simply being within the band doesnot mean that optimum COP will be achieved in any particular case. Thus,optimum efficiency is not achieved at all operating points.

OBJECT OF THE INVENTION

The object of the present invention is to provide an improved airconditioning system that operates at optimum efficiency in all operatingranges. This object is achieved by an air conditioning system as claimedin claim 1.

GENERAL DESCRIPTION OF THE INVENTION

In order to overcome the abovementioned problems, the present inventionproposes an air conditioning system, in particular an air conditioningsystem having carbon dioxide as refrigerant, particularly for use in amotor vehicle. The air conditioning system comprises a compressor, thecompressor having a compressor capacity control element, a gas cooler,an expansion valve and an evaporator arranged in series and forming aclosed circuit for the refrigerant. The air conditioning system furthercomprises a controller for controlling the compressor capacity controlelement and the expansion valve so as to regulate an expansion valveinlet pressure. The controller regulates the expansion valve inletpressure by controlling the compressor capacity control element so as toalign evaporator air off temperature with a set point; by monitoringexpansion valve inlet temperature; by determining a required expansionvalve inlet pressure corresponding to the monitored expansion valveinlet temperature by means of a control algorithm; and by adjusting theexpansion valve and the compressor capacity control element togetheralong an iso-capacity curve to the required expansion valve inletpressure.

By regulating the expansion valve inlet pressure by controlling thecompressor capacity control element so as to align evaporator air offtemperature with a set point, the desired cooling capacity of the systemis set.

By then adjusting the expansion valve and the compressor capacitycontrol element together along an iso-capacity curve to the requiredexpansion valve inlet pressure, the optimum COP can be achieved whilemaintaining that particular cooling capacity. The optimum COP isachieved in all operating conditions due to the use of the algorithm fordetermining the required expansion valve inlet pressure.

Preferably, the control algorithm comprises one or more controlparameters chosen from the list comprising front end air flow; gascooler air inlet temperature; evaporator air flow; evaporator air inlettemperature; evaporator air inlet humidity; compressor speed; and theset point. The inventor has recognised that these control parametersinfluence the efficiency of the system and that it is beneficial to takethem into account in the control algorithm for determining the requiredexpansion valve inlet pressure. Optimum COP can thereby be guaranteed inall operating conditions.

The above control parameters can be set, estimated or measured, e.g.directly measured by means of sensors. The front end air flow can e.g.be estimated as a function of vehicle speed and fan speed. The gascooler air inlet temperature can e.g. be estimated as a function ofvehicle speed and ambient temperature. The evaporator air flow can e.g.be estimated as a function of blower speed, air temperature doorsetting, air distribution mode and air recirculation mode, wherein theair temperature door is used to mix heated air with cooled air so as toobtain the desired air temperature before distributing it to thepassenger compartment, the air distribution mode is used to direct theconditioned air to different parts of the passenger compartments, suchas e.g. windscreen, face or feet, and the air recirculation mode is usedto either draw air from the passenger compartment or from the outside ofthe vehicle. The evaporator air inlet temperature can e.g. be calculatedas a function of cabin temperature, ambient temperature and airrecirculation mode.

According to one embodiment, the compressor is a variable strokecompressor and the compressor capacity control element is a compressorcontrol valve for regulating the stroke of compressor pistons. Such avariable stroke compressor generally comprises a swash plate, swash ringor wobble plate for adjusting the stroke of the compressor pistonsdepending on the pressure acting on the front and the back of thepistons. The compressor control valve is used to regulate the pressureacting on the back of the pistons with respect to the pressure acting onthe front of the pistons, thereby regulating the stroke of the pistonsand hence the capacity of the compressor.

According to another embodiment, the compressor is a variable speedcompressor and the compressor capacity control element is a variablespeed electric drive for regulating the speed of the compressor. Suchcompressors have fixed displacement and use an electric drive to adjustthe speed of the compressor and thereby also the compressor capacity.

Advantageously, an internal heat exchanger is arranged between the gascooler and the expansion valve. The internal heat exchanger removes heatfrom a region between the gas cooler and the expansion device andtransfers it to a region between the evaporator and the compressor. Dueto the internal heat exchanger, more heat can be dissipated in theevaporator, thereby improving the performance and efficiency of thesystem.

It will be appreciated that, if no internal heat exchanger is present,the gas cooler outlet pressure, resp. temperature, is the same as theexpansion valve inlet pressure, resp. temperature. In this case, theexpansion valve inlet pressure, resp. temperature, can be measuredanywhere between the gas cooler and the expansion valve. However, if aninternal heat exchanger is present, there is a temperature differenceand a small pressure difference between the refrigerant at the gascooler outlet and the refrigerant at the expansion valve inlet. Thepressure and temperature should therefore be measured at expansion valveinlet.

DETAILED DESCRIPTION WITH RESPECT TO THE FIGURES

The present invention will be more apparent from the followingdescription of a not limiting embodiment with reference to the attacheddrawing, wherein

FIG. 1 shows a schematic view of an air conditioning system according tothe invention.

FIG. 1 shows an air conditioning system 10, which can e.g. be installedin an automotive vehicle. Such an air conditioning system 10 comprises acompressor 12, a gas cooler 14, an expansion valve 16 and an evaporator18. The different elements 12, 14, 16, 18 are connected in series byfluid pipes and form a closed circuit wherein a refrigerant, e.g. CO₂,can circulate. The refrigerant exits a discharge port 20 of thecompressor 12 under high pressure and is fed to the gas cooler 14, whereit is cooled. The refrigerant then flows to the expansion valve 16,where it expands and drops in pressure. From the expansion valve 16, therefrigerant is led to the evaporator 18, where it evaporates. Therefrigerant is then led back to a suction port 22 of the compressor 12.An internal heat exchanger 24 can be fluidly arranged between an outletport 26 of the gas cooler 14 and an inlet port 28 of the expansion valve16, and between an outlet port 30 of the evaporator 18 and the suctionport 22 of the compressor 12. Such an internal heat exchanger 24,sometimes also referred to as superheater, comprises a high-pressureinternal heat exchanger inlet 32 for receiving refrigerant from the gascooler 14; a high-pressure internal heat exchanger outlet 34 fordelivering refrigerant to the expansion valve 16; a low-pressureinternal heat exchanger inlet 36 for receiving refrigerant from theevaporator 18; and a low-pressure internal heat exchanger outlet 38 fordelivering refrigerant to the compressor 12. The internal heat exchanger24 removes heat from a region between the gas cooler 14 and theexpansion valve 16 and transfers it to a region between the evaporator18 and the compressor 12. More heat can be dissipated in the evaporator18, thereby improving the cooling capacity and efficiency of the airconditioning system 10. Furthermore, an accumulator/dehydrator device 40can be fluidly arranged between the outlet port 30 of the evaporator 18and the suction port 22 of the compressor 12, preferably upstream of theinternal heat exchanger 24. The accumulator/dehydrator device 40prevents liquid refrigerant from reaching the compressor 12 by storingexcess liquid refrigerant coming from the evaporator 18. Theaccumulator/dehydrator device 40 also removes debris and moisture fromthe system.

A controller 42 is provided for controlling the performance of the airconditioning system 10. The controller 42 controls the capacity of thecompressor 12 by acting on a compressor capacity control element (notshown) of the compressor 12. The compressor 12 can be a variable strokecompressor wherein the compressor capacity control element is acompressor control valve. The controller 42 acts on the compressorcontrol valve so as to increase or decrease the pressure of therefrigerant acting on the back of the pistons with respect to thepressure of the refrigerant acting on the front of the pistons, therebyadjusting the stroke of the pistons and the capacity of the compressor12. By changing the capacity of the compressor 12, the cooling capacityof the air conditioning system 10 can be set. Hence, the controller 42is designed to regulate the compressor capacity control elementevaporator pressure so that evaporator air off temperature is alignedwith a set point, i.e. so that the desired cooling capacity is obtained.In order to optimise the COP of the air conditioning system, thecontroller 42 monitors expansion valve inlet temperature; determines arequired expansion valve inlet pressure corresponding to the monitoredexpansion valve inlet temperature by means of a control algorithm andthen adjusts the expansion valve 16 and the compressor capacity controlelement together along an iso-capacity curve to obtain the requiredexpansion valve inlet pressure. The optimum COP can thereby be achievedwhile maintaining that particular cooling capacity.

The controller 42 uses control parameters, such as front end air flow,gas cooler air inlet temperature, evaporator air flow; evaporator airinlet temperature, evaporator air inlet humidity, compressor speed andthe set point, in the control algorithm to determine the requiredexpansion valve inlet pressure. The inventor has recognised that, ashead pressure is changed, by whatever means, there are associatedchanges in just about every other operating parameter within therefrigerant loop and significant deviation from the idealisation isseen. For example, as the expansion valve is closed to increase headpressure, mass flow rate changes, heat exchanger effectiveness and thusfluid outlet conditions adjust and isentropic efficiency of thecompressor changes as pressure ratio changes. The gas cooler outlettemperature and evaporator pressure are therefore not sufficient todetermine the required expansion valve inlet pressure since some or allof the above control parameters influence the efficiency of the system.It is thus beneficial to take the above-mentioned control parametersinto account in the control algorithm for determining the requiredexpansion valve inlet pressure. Optimum COP can then be guaranteed inall operating conditions.

The inventor has also recognised that each operating point optimum hasits own tolerance band. It will be appreciated that including everysingle control parameter in the algorithm can lead to the need for quitesubstantial computing power, which is not necessarily available in avehicle. By using only some of the control parameters, the neededcomputing power can be reduced, while at the same time, the accuracy ofthe COP point is reduced. A compromise can hence be made between theneeded computing power and the accuracy with which the requiredexpansion valve inlet pressure can be determined for optimum COP.

REFERENCE SIGNS

-   10 air conditioning system-   12 compressor-   14 gas cooler-   16 expansion valve-   18 evaporator-   20 discharge port of compressor-   22 suction port of compressor-   24 internal heat exchanger-   26 outlet port of gas cooler-   28 inlet port of expansion valve-   30 outlet port of evaporator-   32 high-pressure internal heat exchanger inlet-   34 high-pressure internal heat exchanger outlet-   36 low-pressure internal heat exchanger inlet-   38 low-pressure internal heat exchanger outlet-   40 accumulator/dehydrator device-   42 controller

1. Air conditioning system, in particular air conditioning system havingcarbon dioxide as refrigerant, particularly for use in a motor vehicle,said air conditioning system comprising a compressor, a gas cooler, anexpansion valve and an evaporator arranged in series and forming aclosed circuit for said refrigerant; said compressor having a compressorcapacity control element and said air conditioning system furthercomprising a controller for controlling said compressor capacity controlelement and said expansion valve so as to regulate an expansion valveinlet pressure; said controller regulating said expansion valve inletpressure by controlling said compressor capacity control element so asto align evaporator air off temperature with a set point; monitoringexpansion valve inlet temperature; determining a required expansionvalve inlet pressure corresponding to said monitored expansion valveinlet temperature by means of a control algorithm; and adjusting saidexpansion valve and said compressor capacity control element togetheralong an iso-capacity curve to said required expansion valve inletpressure.
 2. System according to claim 1, wherein said control algorithmcomprises one or more control parameters chosen from the listcomprising: front end air flow; gas cooler air inlet temperature;evaporator air flow; evaporator air inlet temperature; evaporator airinlet humidity; compressor speed; and set point.
 3. System according toclaim 2, wherein said control parameters are set, estimated or measured.4. System according to claim 2 or 3, wherein said front end air flow isestimated as a function of vehicle speed and fan speed.
 5. Systemaccording to any of claims 2 to 4, wherein said gas cooler air inlettemperature is estimated as a function of vehicle speed and ambienttemperature.
 6. System according to any of claims 2 to 5, wherein saidevaporator air flow is estimated as a function of blower speed, airtemperature door setting, air distribution mode and air recirculationmode.
 7. System according to any of claims 2 to 6, wherein saidevaporator air inlet temperature is estimated as a function of cabintemperature, ambient temperature and air recirculation mode.
 8. Systemaccording to any of claims 1 to 7, wherein said compressor is a variablestroke compressor and said compressor capacity control element is acompressor control valve.
 9. System according to any of claims 1 to 7,wherein said compressor is a variable speed compressor and saidcompressor capacity control element is a variable speed electric drive.10. System according to any of claims 1 to 9, wherein an internal heatexchanger is arranged between said gas cooler and said expansion valve.